This invention relates generally to liquid crystal devices and particularly to an alignment layer for such devices, and to a method of forming such layers.
Display devices which utilize twisted nematic liquid crystals include a liquid crystal material disposed between two insulative substrates, at least one of which is transparent. Electrical control electrodes are disposed on the surfaces of the substrates which face the liquid crystal material. In one type of liquid crystal device both the substrates and their associated electrodes are transparent to permit light to pass through the entire structure when the elongated molecules of the liquid crystal material are in one orientation, or to be opaque to light for another orientation of the liquid crystal molecules. In another type of liquid crystal device only one substrate and its associated electrode are transparent. In this type of device, light passes through the substrate and is reflected off the liquid crystal material for one state of the liquid crystal material. For another state of the liquid crystal material, light passes through the liquid crystal material and is reflected from the other substrate For both types of devices images are formed by voltage biasing the control electrodes of selected cells to change the light transmission capabilities of the energized cells.
In both types of liquid crystal devices the molecules of the liquid crystal material are elongated and must be properly aligned with the surfaces of the substrates in order to assure uniformity of operation and high contrast of the displayed information. The required alignment of the liquid crystal molecules is achieved by coating the surfaces of the electrodes with an alignment material and rubbing the alignment material in the direction desired for the alignment of the molecules.
In active matrix liquid crystal display devices a switching device, such as a thin film transistor (TFT) or a solid state diode, is associated with each of the liquid crystal elements within the display. The alignment layer for active matrix liquid crystal devices must control three properties to provide optimum operation and contrast. The properties are the tilt angle between the principal axis of the molecules and the substrate, molecular alignment (parallel orientation of the liquid crystal molecules), and a high RC time constant. The tilt angle should be between 1.degree. and 5.degree. to provide a suitable response time and to optimize the viewability of the device. Good molecular alignment of the liquid crystal molecules is necessary to provide uniformity of operation and high contrast between the energized and deenergized states. A high RC time constant is needed because the resistivity of the liquid crystal material must be sufficiently high to assure that the liquid crystal cells are capable of storing a charge for a time period of sufficient duration to display the desired image. It is desirable for the RC time constant to be in excess of 100 msec, it is also desirable that the time constant does not change during life of the display device.
In directly driven liquid crystal devices, i.e. those which do not employ solid state switching devices, the alignment layer must control only the tilt angle and the alignment of the liquid crystal molecules. The time constant, or resistivity, of the liquid crystal material is of no concern, because the liquid crystal cells are not called upon to store a charge during the display of information. Accordingly, the alignment layer for directly driven display devices frequently is polyimide. The polyimide provides excellent molecular alignment and a desirable tilt angle within the range of 1.5.degree. to 3.degree.. Initially, the time constant of a cell having a polyimide alignment layer is 30 to 100 msec. Upon exposure to elevated temperatures, in the order of 90.degree. C., the time constant degrades to 10 to 30 msec. A time constant of this duration is unacceptable for active matrix liquid crystal displays. Also, polyimide alignment material frequently degrades the resistivity of commonly used liquid crystal materials and, therefore, is unsuitable as an alignment layer in active matrix liquid crystal devices. Other materials, which do not adversely effect the resistivity of the liquid crystal material, typically demonstrate either poor molecular alignment or inadequate tilt angle and, therefore, also are not useful as alignment materials for active matrix liquid crystal devices. For example, when angle evaporated SiO.sub.2 is used as the alignment material the tilt angle is difficult to control because the deposition is not easily repeatable.
For these reasons there is a need for an alignment layer for active matrix liquid crystal devices which provides acceptable tilt angle, good alignment of the liquid crystal molecules and a RC time constant suitable for use with active matrix liquid crystal devices. The present invention fulfills these needs.